Boundary-layer and aerosol/cloud interaction in central Arctic summer observed during ASCOS

Low-level clouds play a decisive role for the surface energy budget and for the central Arctic represent a surface warming most of the year. This is partly due to a relatively high surface albedo and large solar zenith angles. These clouds are often optically thin, making them sensitive to changes in for example surface fluxes, natural as well as anthropogenic aerosol concentration and prevailing weather conditions. Models have great difficulty describing these clouds in a reasonable fashion; it has been suggested that the large inter-model scatter in Arctic climate change scenarios is to large extent due to differences in cloud descriptions in climate models. If this sensitivity is incorrect in models, climate change scenarios for the Arctic are not reliable.

We will describe a study from the central Arctic in summer using data collected during the Arctic Summer Cloud Ocean Study (ASCOS, www.ascos.se). The ASCOS field experiment was deployed on the Swedish icebreaker Oden north of 87°N in the Atlantic sector of the Arctic. The data used in this study was collected during a three-week ice drift with the Oden moored to a drifting multi-year ice floe 12 August – 2 September, when intensive measurements were taken on the ice and onboard. In addition to regular weather observation, 6-hourly radiosoundings, detailed cloud observation from the MMCR cloud radar and winds from a 449 MHz wind profiler onboard, we also use surface radiation, surface and ice temperatures, eddy-correlation fluxes, wind observations from a SODAR all deployed on the ice. We will first demonstrate the effects of low clouds on the surface energy balance and elaborate on the linkages between the clouds, the surface and aerosols. The issue of if the clouds are connected to the boundary layer is important to the conceptual idea of the source areas for the aerosols necessary for cloud formation in the first place.

We will demonstrate that the idea of surface coupling has more than one facet and that this coupling via radiative processes can be as strong as that by turbulent fluxes and how these ideas can be used to define melt and freeze regimes. We will also show how the presence of the clouds is sometimes limited by the absence of a sufficient number of cloud condensation nuclei and how this can potentially feeds back to changes in surface radiative forcing from the clouds by changes in the aerosol concentration.